Method for improving conditions in closed circuit fish farming

ABSTRACT

The invention relates to a method for improving conditions in closed circuit seafood and fish farming, comprising the following steps: a) at least part of the water from at least one of the rearing ponds ( 1 ) is recycled, whereby said water is treated one or more times and purified ( 3, 4, 5 ) before it is re-injected into said pond; b) the water recycled during the recycling path indicated in step a) is mixed with a new supply of water ( 10 ); c) all or part of the recycled water ( 21, 22 ) undergoes ozonization and, wherever applicable, the new supply of water ( 20 ) also undergoes ozonization either partially or in full before it is mixed with the re-circulated water.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of aquaculture, in particularbut not exclusively in seawater, an industrial sector in fulldevelopment, especially for the production of sea bass, sea bream andturbot.

It is aimed in particular at closed circuit fish farming.

2. Description of the Related Art

Considering here the case of seawater fish farming, it is in fact knownthat seawater fish farms operate in an open circuit discharging verylarge quantities of contaminated water. Typical quantities of waterneeded and discharged are assessed at about 200 to 300 m³ water/kg offish produced. It is then understood that depending on the site at whichsuch farms are installed, increasing environmental constraints are nowtending to require the treatment of discharged waters, or even anabsolute limit on the amount of water discharged.

Such a technique of farming seawater fish in a closed circuit (where atleast part of the seawater of the rearing pond or ponds is thereforerecycled in order to purify it and reinject it into the pond inquestion), is unquestionably an emerging technique in full development,even if its control is, in practice, very difficult and precise since itrequires specific treatments to control the pH, the oxygen content, theCO₂ content, the microbial and particulate load, the nitrogen load(especially aqueous ammonia), the dissolved contaminants, or even thecolor of the water in the ponds.

The fact remains that depending on the fish farmers, the control of thistechnique of closed circuit farming may enable, depending on the case,from 10 to 95 or even 98% of the seawater to be recycled, the rest beingprovided by new seawater makeup.

Nevertheless, it appears in practice, that although closed circuit watertreatment circuits, which are quite complex, make it possible to controleffectively parameters such as the pH, dissolved gases or even themicrobial load of the recycled water (use of mechanical filters, ofbiological filters, of degassing columns, etc.), other problems are notyet truly resolved. Among these technical problems, which constitutereal thorns in the life of these aquaculture plants, there is theproblem of water coloration.

Specifically, the appearance of a yellow coloring of the water isobserved, particularly at high recirculation rates (above 80%), whichlimits the visibility and unquestionably leads to stress in the farmedspecies.

Undoubtedly, the nature of these substances has not really beenidentified to date, but some works incline by towards the interventionof humic acids in this phenomenon. It is then found that no technologyhas really been validated in the current literature in order to resolvethis problem, a problem which is most often termed “yellow water”, UVlight in particular having no effect on this color phenomenon.

SUMMARY OF THE INVENTION

One of the objects of the present invention is then especially toprovide a method making it possible to improve the conditions in closedcircuit fish farming, whether it involves seawater fish or freshwaterfish or other sea products, aiming especially to demonstrate, on thephenomenon described above, a curative effect (in order to substantiallyremove the substances which would already have accumulated) and apreventive effect (in order to substantially limit the appearance ofthese coloring substances), while of course maintaining conditionsacceptable for the farmed species in terms of toxicity and microbiology.

To simplify the terminology, in the following, the term “fish” will beused in the knowledge that it covers, according to the invention (unlessotherwise specified) fish, eggs, seawater or freshwater young fish, oreven sea products such as shellfish and mollusks.

The invention therefore relates to a method for improving conditions inclosed circuit fish farming, in which:

a) at least part of the water of at least one rearing pond is recycled,recycled water which undergoes one or more purification treatments,before being reinjected into the pond in question;

b) in the recycling path of step a), the recycled water is mixed withnew water makeup;

c) ozonation of all or part of the recycled water is carried out and inaddition, if necessary, ozonation of the new water makeup is carried outbefore mixing it in step b.

The method for improving farming conditions according to the inventionwill also adopt one or more of the following characteristics:

the mixing of the recycled water and the new water makeup is carried outin a retention tank;

the new water makeup is ozonated by injecting a gas comprising ozone, soas to obtain an ozone content in the water of between 1 and 15 mgozone/liter, and more preferably, between 2 and 10 mg ozone/liter ofwater;

the recycled water which is ozonated is ozonated by injecting a gascomprising ozone, so as to obtain an ozone content in the recycled waterof between 0.05 and 2 mg ozone/liter, and more preferably between 0.1and 1 or even 0.5 mg ozone/liter of recirculated water;

ozonation of all or part of the recirculated water and of the new watermakeup is carried out;

the recycling path comprises a purification treatment using a biologicalfilter, and the gas comprising ozone is then injected upstream of thisbiological filter;

the recycling path comprises a purification treatment using a biologicalfilter, and the gas comprising ozone is then injected downstream of thisbiological filter, before the recirculated water is mixed with the newwater makeup;

ozonation of all or part of the recirculated water and of the new watermakeup is carried out, and two ozonators are available for this purposeso that each prepares one of the two ozonation gases needed;

ozonation of all or part of the recirculated water and of the new watermakeup is carried out, and a single ozonator is available for thispurpose, operating according to the following procedures:

i) said ozonator produces a main flow of ozonated gas;

j) a predetermined portion of the main flow of ozonated gas is divertedto a calibrated orifice, in order to direct it toward the recirculatedwater stream to be ozonated;

k) the power of said ozonator is adjusted on the basis of the flow rateof new water to be ozonated.

ozonation of all or part of the recirculated water and of the new watermakeup is carried out, and a single ozonator is available for thispurpose, operating according to the following procedures:

i) said ozonator supplies a main flow of ozonated gas;

j) a predetermined portion of the main flow of ozonated gas is divertedthrough a calibrated orifice, in order to direct it toward the new waterto be ozonated;

k) the power of said ozonator is adjusted on the basis of the flow rateof recirculated water to be ozonated.

the improved fish farming is closed circuit farming of seawater fish.

The invention also relates to a method for decoloring the water of aclosed circuit fish farm, a farm of the type where at least part of thewater of at least one rearing pond is recycled, recycled water whichundergoes one or more purification treatments before being reinjectedinto the pond in question, while in the recycling path, the recycledwater is mixed with new water makeup in a retention tank, characterizedin that the ozonation of all or part of the recirculated water, and alsoif necessary the ozonation of the new water makeup, is carried outbefore it is mixed with the recycled water.

The decoloration method according to the present invention must beunderstood as aiming in particular to limit the phenomenon of theappearance of yellow water, and to substantially eliminate thesubstances which would have accumulated beforehand.

The decoloration method according to the invention will moreover be ableto adopt one or more of the following characteristics:

mixing of the recycled water and the new water makeup is carried out ina retention tank;

the new water makeup is ozonated by injection of a gas comprising ozone,so as to obtain an ozone content in the water of between 1 and 15 mgozone/liter, and more preferably between 2 and 10 mg ozone/liter of newwater, while the recirculated water is ozonated by injection of a gascomprising ozone so as to obtain an ozone content in the recirculatedwater of between 0.05 and 2 mg ozone/liter, and more preferably between0.1 and 0.5 mg ozone/liter of recirculated water;

the recycling path comprises a purification treatment using a biologicalfilter, and the ozonated gas is then injected upstream of thisbiological filter;

the recycling path comprises a purification treatment using a biologicalfilter, and the ozonated gas is then injected downstream of thisbiological filter, before the recirculated water is mixed with the newwater makeup;

ozonation of all or part of the recirculated water and of the new watermakeup is carried out, a single ozonator being available for thispurpose, which operates according to the following procedures:

i) said ozonator produces a main flow of ozonated gas;

j) a predetermined portion of the main flow of ozonated gas is divertedthrough a calibrated orifice, in order to direct it toward therecirculated water to be ozonated;

k) the power of said ozonator is adjusted on the basis of the flow rateof new water to be ozonated.

ozonation of all or part of the recirculated water and of the new watermakeup is carried out, a single ozonator being available for thispurpose, which operates according to the following procedures:

i) said ozonator produces a main flow of ozonated gas;

j) a predetermined portion of the main flow of ozonated gas is divertedthrough a calibrated orifice, in order to direct it toward the new waterto be ozonated;

k) the power of said ozonator is adjusted on the basis of the flow rateof recirculated water to be ozonated.

the fish farming treated in this way is closed circuit farming ofseawater fish.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING

Other characteristics and advantages of the invention will emerge fromthe following description, given solely by way of illustration and in noway limiting, and made with reference to the appended drawings in which:

FIG. 1 is a schematic representation of an exemplary closed circuitseawater fish farming plant;

FIG. 2 is a schematic representation of another typical exemplary closedcircuit seawater fish farming plant, used for the implementation ofexemplary embodiments according to the invention as detailed below inthe present application;

FIG. 3 is a partial schematic representation of an exemplary systemallowing, for example, the ozonation of water recirculated in the loop;

FIG. 4 is a partial representation of an exemplary system allowing theozonation of the new water makeup;

FIG. 5 is a schematic illustration of the case of a single ozonator usedto ozonate both the recirculated water and the new water makeup.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates the presence of a pond (1) for rearing fish, in thiscase, seawater fish (one rearing pond or of course one of the rearingponds of the site in question), of which at least some of the seawateris recycled, via the recirculation line 2.

The seawater recirculated in this way undergoes in its path a number ofpurification steps, among which could be passage through a mechanicalfilter 3, passage through a biological filter 4 (nitrifying thendenitrifying), together with a degassing column (5) making it possibleespecially to remove the carbon dioxide or even the dissolved nitrogen.

Moreover, note at the top of the figure a new water makeup (via the line10 shown in the figure as a bold line), which passes for example, beforereaching a retention tank 6, via a mechanical filtration step 11, andvia a heat exchanger 12 in order to bring as required the new watermakeup to the optimum temperature for the fish to grow.

The recirculated water coming from the column 5 is mixed with the newwater coming from the retention tank 6, in order then to return to thepond 1.

Such a closed circuit may typically allow from 10 to 95% or even 98% ofthe seawater in the ponds in question to be recycled.

FIG. 2 shows a variant of a closed circuit fish farming plant, in thiscase also using seawater, in which the presence of a rearing tank 1 willbe seen, from which at least part of the seawater is recycled to aretention tank 6, also fed moreover by new seawater makeup (line 10shown here again as a bold line).

It is therefore here, within the scope of the present variant, that theseawater from the retention tank 6 which undergoes various purificationsteps including mechanical filtration 3, biological filtration 4, oreven the degassing column 5, is mixed before being used in the pond 1.

On the lines, exemplary locations of ozonated gas injection according tothe invention (20, 21, 22), have been shown, as rectangles drawn asdotted lines, on the new water makeup line downstream of the mechanicalfiltration system, and on the recycling line coming from the retentiontank 6 upstream and downstream of the biological filter 4.

FIGS. 3 and 4 specifically show two exemplary embodiments of theinjection of ozonated gas into the water of the loop and into the newwater makeup.

Thus FIG. 3 shows a possible embodiment of the injection of ozonated gasat the location 22, downstream of the biological filter 4, ozonation ofthe water of the closed circuit coming from the retention tank 6.Although in the following, the embodiment of FIG. 3 is preferred forozonating the recirculated water and that of FIG. 4 for ozonating thenew water, it is understood that this is technically completelyinterchangeable, and that therefore for example the system of FIG. 3 canalso be used to ozonate the new water.

Thus in FIG. 3, the inflow of recirculated water from the biologicalfilter 4 may be seen, opening out in the top part of an ozonation column30, in the bottom of which there is a system for injecting ozonated gasinto the water of the column (in this case two porous systems 32 and33), which injection system is connected to a pipe 31 for the inflow ofozonated gas.

An air intake pipe 34 is present in the top of the column 30, while apipe for removing ozonated water is provided at the bottom of the columnin order to direct the water thus ozonated to the following step whichhappens to be, for the embodiment shown, the degassing column 5.

It is understood then that such an arrangement allowing a counterstreamof ozonated gas bubbles and of the inflow of water from the closedcircuit enables the water flow to be ozonated under simple andeconomical conditions, a structure that will be quite particularlyfavored in the case of relatively low ozonation rates, preferably below1 ppm.

FIG. 4 provides another exemplary embodiment of the ozonation of astream of water, typically suitable for the ozonation point 20 of FIG. 2(ozonation of new water makeup), and the figure shows the pipe 10 forthe inflow of new water makeup, coming from the filtration system 11, astream of new water which encounters, in a mixer 40 (for example: coil,static mixer, contact tower, venturi-type hydroinjector, etc.), aninflow 41 of gas comprising ozone coming from an ozonator.

The stream of new water thus ozonated is then directed to a storage tank42 ensuring a suitable contact time, and equipped in a conventionalmanner with a gas discharge system 43 (discharge to a conventionalsystem for recovery, detoxification, etc.).

Of course it is then possible according to this embodiment to take waterthus ozonated from the tank 42 in order to direct the water in this caseto the retention tank 6, where it will be mixed with the recirculatedwater from the pond 1.

As for FIG. 5, it illustrates a very advantageous embodiment of theinvention, where both the new water makeup and the recirculated waterare ozonated in the closed circuit, this by means of a single ozonator(reference 50 in the figure), the ozonator operating according to thefollowing procedures:

FIG. 5 shows the new water makeup pipe 10 and its point 20 for injectingozonated gas, the pipe 10 being, in this detailed view, equipped with asystem 54 for measuring the water flow rate;

the pipe 2 for recirculating water coming from the retention tank 6, inthis case detailed at the point of injection 22 of ozonated gas, is alsoshown;

in order to ozonate the two aforementioned locations of the circuitunder satisfactory conditions, the single ozonator 50 therefore supplies(pipe 51), a main flow of ozonated gas, a predetermined portion of theflow of which is diverted (pipe branch 52) via a calibrated orifice 53in order to direct this ozonated gas to the location 22 (closedcircuit). The rest of the main flow of ozonated gas (line 51) isdirected to the ozonation point 20 (new water);

a data acquisition and processing unit such as a programmable controller60 then receives from the flow meter 54 information concerning the flowof new water to be treated in the pipe 10, and then retroacts accordingto this information in order to adjust the power of the ozonator 50according to the variation in water flow in the pipe 10 (it is knownthat this flow of new water may in particular vary within a certainrange depending on maintenance operations on filters and other pumpsupstream);

it can then be seen that the unit 60 will, depending on the variation innew water flow rate in the pipe 10, vary the power of the ozonator 50,while the flow of ozonated gas in the pipe 51 is itself substantiallyconstant, which will therefore vary the ozone content in this main flowof ozonated gas;

according to this embodiment, choosing to place a calibrated orifice(53) on the branch 52 will then give rise to the constant flow ofozonated gas to the point 22, the ozone content of which will howeveralso vary within a predetermined range;

hence it can be seen that by a suitable adjustment of the power of theozonator and of the calibrated orifice (53), it will be possible todeliver ozonated gas within the content range required by the user siteto both the location 20 and to the location 22.

Such an arrangement (a single ozonator, calibrated orifice, etc.),therefore represents a structure which is light and very economical, butwhich supplies the required performance to the various points of thecircuit, such as for example a content between 2 and 10 mg ozone/literof new water to the point 20, and between 0.1 and 0.5 mg ozone/liter ofrecirculated water to the point 22.

A plant such as that shown within the scope of FIGS. 2 and 5 was usedfor examples of implementing the invention, under the followingoperating conditions:

consider a pond (1) for rearing seawater fish, operating with 30 to 100m³ of new water makeup, and a recirculation of about 1000 m³/h waterfrom pond 1;

an ozonation plant is used with a single ozonator as shown within thescope of FIG. 5, to set up the following ozone content, namely about 3ppm ozone in the new water and a content close to 0.1 ppm in the waterof the circuit at the point 22, that is to say downstream of thebiological filter 4;

as indicated above, the ozonator will then see its power vary within arange of about 1.8 to 3.2 g ozone per m³, while the ozone content in theline 52 will typically vary between 0.07 and 0.13 ppm;

the effect of such treatment on the quality of the water throughout, thecircuit is then evaluated, firstly for the bacteria content per ml waterand secondly, for the absorbance of the water at 400 nanometers (becauseof the yellow water phenomenon mentioned above);

it is of course understood that all these results are monitored withrespect to a control pond operating normally in a closed circuit butwithout any inflow of ozonated gas according to the invention.

It is then possible to summarize the very spectacular results obtainedby the following data:

a) it is noticed firstly, that the absorbance of the water is at leasthalved compared to the control pond, which perfectly confirms theeveryday visual perception of the farmer with respect to the clarity ofthe water;

b) a reduction in the flowing bacterial content of the order of one logis obtained;

c) the farmer notices unquestionable efficiency of the ozone treatmentaccording to the invention both on the decoloration of closed circuitsand on the quality of the water in general, leading from his point ofview even to an improvement in the biomass of fish compared with thecontrol pond, in proportions which remain to be determined from tests inthe longer term.

All these results are all the more spectacular and satisfactory sinceseawater is a complex and very reactive medium, which the ozone oxidizesnonselectively thereby forming oxidation byproducts which the literaturecalls “total residual oxidants”, the nature of which has not beencompletely clarified but which are known to be toxic for fish when thepresence of the oxidants is not adequately controlled.

Hence the fundamental importance of controlling, according to theinvention, the water which is ozonated, the points of injecting ozonatedgas, the latent and retention times and the dose rates, so as also tocontrol the total residual oxidants. It will have been noted for examplethat the dose rates proposed for the new water makeup are substantiallyincreased compared with the dose rates that the person skilled in theart of fish farming currently has in mind.

Although the present invention has been described in relation toparticular embodiments, it is not limited thereby in any way, but is onthe contrary capable of modifications and variants which will appear tothe person skilled in the art within the scope of the claims below.

Thus, for example, although, according to one of the aspects of theinvention, the ozonation of the new water flow has been mentioned above,it would be possible to ozonate only part of the new water flow reachingthe ponds without departing at any time from the scope of the presentinvention.

What is claimed is:
 1. A method for improving farming conditions inclosed circuit fish farming, comprising: a) recycling at least part ofthe water of at least one rearing pond, said recycled water undergoingone or more purification treatments, before being reinjected into thepond in question; b) mixing the recycled water with new water makeup; c)ozonating at least part of the recycled water and ozonating at leastpart of the new water makeup before mixing it with the recycled water,wherein a single ozonator is present as the sole ozonator in the circuitand operates according to the following procedures: said ozonatorproduces a main flow of ozonated gas; a predetermined portion of themain flow of ozonated gas is directed to a calibrated orifice, in orderto direct a constant flow toward the recycled water; the output of saidozonator being adjusted on the basis of the flow rate of the new waterto be ozonated.
 2. The method for improving farming conditions asclaimed in claim 1, wherein the new water is ozonated to obtain an ozonecontent in the water of between 1 and 15 mg ozone/liter.
 3. The methodof claim 2, wherein the new water is ozonated to obtain an ozone contentof between 2 and 10 mg ozone/liter of water.
 4. The method for improvingfarming conditions as claimed in claim 2, wherein the recycling pathincludes a purification treatment using a biological filter, and theozonated gas is then injected upstream of this biological filter.
 5. Themethod for improving farming conditions as claimed in claim 2, whereinthe recycling path includes a purification treatment using a biologicalfilter, and the ozonated gas is then injected downstream of thisbiological filter, before the recirculated water is mixed with the newwater makeup.
 6. The method for improving farming conditions as claimedin claim 2, wherein the recycled water and the new water makeup aremixed in a retention tank.
 7. The method for improving farmingconditions as claimed in claim 2, wherein the fish farming improved inthis way is closed circuit farming of seawater fish.
 8. The method forimproving farming conditions as claimed in claim 1, wherein therecirculated water is ozonated by to obtain an ozone content in therecirculated water of between 0.05 and 2 mg ozone/liter.
 9. The methodof claim 8, wherein the recirculated water is ozonated to obtain anozone content between 0.1 and 0.5 mg ozone/liter of water.
 10. Themethod for improving farming conditions as claimed in claim 1, whereinthe recycling path includes a purification treatment using a biologicalfilter, and the ozonated gas is then injected upstream of thisbiological filter.
 11. The method for improving farming conditions asclaimed in claim 1, wherein the recycling path includes a purificationtreatment using a biological filter, and the ozonated gas is theninjected downstream of this biological filter, before the recirculatedwater is mixed with the new water makeup.
 12. The method for improvingfarming conditions as claimed in claim 1, wherein the recycled water andthe new water makeup are mixed in a retention tank.
 13. The method forimproving farming conditions as claimed in claim 1, wherein the fishfarming improved in this way is closed circuit farming of seawater fish.14. A method for decoloring the water of a closed circuit fish farm,which comprises: recycling at least part of the water of at least onerearing pond, subjecting said recycled water to one or more purificationtreatments before being reinjected into the pond in question, and mixingthe recycled water with a new water makeup, wherein ozonation of atleast part of the recycled water and ozonation of at least part of thenew water makeup is carried out with a single ozonator present as thesole ozonator in the circuit and operating according to the followingprocedures: said ozonator produces a main flow of ozonated gas; apredetermined portion of the main flow of gas is directed through acalibrated orifice, in order to direct a constant flow toward therecycled water to be ozonated; and the output of said ozonator isadjusted on the basis of the flow rate of new water to be ozonated. 15.The decoloration method as claimed in claim 14, wherein the followingozonation dose rates are adopted: the new water makeup is ozonated toobtain an ozone content in the water of between 1 and 15 mg ozone/liter;the recirculated water is ozonated to obtain an ozone content in therecirculated water of between 0.05 and 2 mg ozone/liter.
 16. The methodof claim 15, wherein the new water is ozonated to an ozone content ofbetween 2 and 10 mg ozone/liter of water and the recirculated water isozonated to an ozone content between 0.1 and 0.5 mg/liter of water. 17.The decoloration method as claimed in claim 15, wherein the recyclingpath includes a purification treatment using a biological filter, andthe ozonated gas is then injected upstream of this biological filter.18. The decoloration method as claimed in claim 15, wherein therecycling path includes a purification treatment using a biologicalfilter, and the ozonated gas is then injected downstream of thisbiological filter, before the recirculated water is mixed with the newwater makeup.
 19. The decoloration method as claimed in claim 15,wherein the mixing of the recycled water and the new water makeup iscarried out in a retention tank.
 20. The decoloration method as claimedin claim 15, wherein the fish farming treated in this way is closedcircuit farming of seawater fish.
 21. The decoloration method as claimedin claim 14, wherein the recycling path includes a purificationtreatment using a biological filter, and the ozonated gas is theninjected upstream of this biological filter.
 22. The decoloration methodas claimed in claim 14, wherein the recycling path includes apurification treatment using a biological filter, and the ozonated gasis then injected downstream of this biological filter, before therecirculated water is mixed with the new water makeup.
 23. Thedecoloration method as claimed in claim 14, wherein the mixing of therecycled water and the new water makeup is carried out in a retentiontank.
 24. The decoloration method as claimed in claim 14, wherein thefish farming treated in this way is closed circuit farming of seawaterfish.
 25. A plant for decoloring the water of a closed circuit fishfarm, which comprises means for recycling at least part of the water ofat least one rearing pond, means for purifying the recycled water beforeits reuse in the pond in question, new water makeup means for additionto the stream of recycled water, means for ozonating at least part ofthe recycled water and at least part of the new water makeup before itis mixed with the recycled water, said means for ozonating comprising: asingle ozonator as the sole ozonator in the circuit and capable ofsupplying a main flow of ozonated gas; a line for directing apredetermined portion of the main flow of ozonated gas produced by theozonator, through a calibrated orifice, in order to direct a constantflow toward the recycled water to be ozonated; means for measuring theflow rate of new water to be ozonated; and means for adjusting theoutput of said ozonator on the basis of the flow rate of the new watermeasured by said measuring means.
 26. The plant as claimed in claim 25,wherein the fish farm treated in this way is a closed circuit farm forseawater fish.
 27. A method for improving farming conditions in closedcircuit fish farming, comprising: a) recycling at least part of thewater of at least one rearing pond, said recycled water undergoing oneor more purification treatments, before being reinjected into the pondin question; b) mixing the recycled water with new water makeup; c)ozonating at least part of the recycled water and ozonating at leastpart of the new water makeup before mixing it with the recycled water,wherein a single ozonator is present as the sole ozonator in the circuitand operates according to the following procedures: said ozonatorproduces a main flow of ozonated gas; a predetermined portion of themain flow of ozonated gas is directed to a calibrated orifice, in orderto direct a constant flow toward the new water to be ozonated; theoutput of said ozonator being adjusted on the basis of the flow rate ofthe recycled water to be ozonated.
 28. The method for improving farmingconditions as claimed in claim 27, wherein the new water is ozonated toan ozone content in the water of between 1 and 15 mg ozone/liter and therecirculated water is ozonated to an ozone content in the recirculatedwater of between 0.05 and 2 mg ozone/liter.
 29. The method of claim 28,wherein the new water is ozonated to an ozone content between 2 and 10mg ozone/liter of water and the recirculated water is ozonated to anozone content between 0.1 and 0.5 mg ozone/liter of water.
 30. Themethod for improving farming conditions as claimed in claim 28, whereinthe recycling path includes a purification treatment using a biologicalfilter, and the ozonated gas is then injected upstream of thisbiological filter.
 31. The method for improving farming conditions asclaimed in claim 28, wherein the recycling path includes a purificationtreatment using a biological filter, and the ozonated gas is theninjected downstream of this biological filter, before the recirculatedwater is mixed with the new water makeup.
 32. The method for improvingfarming conditions as claimed in claim 28, wherein the recycled waterand the new water makeup are mixed in the retention tank.
 33. The methodfor improving farming conditions as claimed in claim 28, wherein thefish farming improved in this way is closed circuit farming of seawaterfish.
 34. The method for improving farming conditions as claimed inclaim 27, wherein the recycling path includes a purification treatmentusing a biological filter, and the ozonated gas is then injectedupstream of this biological filter.
 35. The method for improving farmingconditions as claimed in claim 27, wherein the recycling path includes apurification treatment using a biological filter, and the ozonated gasis then injected downstream of this biological filter, before therecirculated water is mixed with the new water makeup.
 36. The methodfor improving farming conditions as claimed in claim 27, wherein therecycled water and the new water makeup are mixed in a retention tank.37. The method for improving farming conditions as claimed in claim 27,wherein the fish farming improved in this way is closed circuit farmingof seawater fish.
 38. A method for decoloring the water of a closedcircuit fish farm, which comprises: recycling at least part of the waterof at least one rearing pond, subjecting said recycled water to one ormore purification treatments before being reinjected into the pond inquestion, and mixing the recycled water with a new water makeup, whereinozonation of at least part of the recycled water and ozonation of atleast part of the new water makeup is carried out with a single ozonatorpresent as the sole ozonator in the circuit and operating according tothe following procedures: said ozonator produces a main flow of ozonatedgas; a predetermined portion of the main flow of gas is directed througha calibrated orifice, in order to direct a constant flow toward the newwater to be ozonated; and the output of said ozonator is adjusted on thebasis of the flow rate of recycled water to be ozonated.
 39. Thedecoloration method as claimed in claim 38, wherein the followingozonation dose rates are adopted: the new water makeup is ozonated toobtain an ozone content in the water of between 1 and 15 mg ozone/liter;the recirculated water is ozonated to obtain an ozone content in therecirculated water of between 0.05 and 2 mg ozone/liter.
 40. The methodof claim 39, wherein the new water is ozonated to an ozone content ofbetween 2 and 10 mg ozone/liter of water and the recirculated water isozonated to an ozone content between 0.1 and 0.5 mg/liter of water. 41.The decoloration method as claimed in claim 39, wherein the recyclingpath includes a purification treatment using a biological filter, andthe ozonated gas is then injected upstream of this biological filter.42. The decoloration method as claimed in claim 39, wherein therecycling path includes a purification treatment using a biologicalfilter, and the ozonated gas is then injected downstream of thisbiological filter, before the recirculated water is mixed with the newwater makeup.
 43. The decoloration method as claimed in claim 39,wherein the mixing of the recycled water and the new water makeup iscarried out in a retention tank.
 44. The decoloration method as claimedin claim 39, wherein the fish farming treated in this way is closedcircuit farming of seawater fish.
 45. The decoloration method as claimedin claim 38, wherein the recycling path includes a purificationtreatment using a biological filter, and the ozonated gas is theninjected upstream of this biological filter.
 46. The decoloration methodas claimed in claim 38, wherein the recycling path includes apurification treatment using a biological filter, and the ozonated gasis then injected downstream of this biological filter, before therecirculated water is mixed with the new water makeup.
 47. Thedecoloration method as claimed in claim 38, wherein the mixing of therecycled water and the new water makeup is carried out in a retentiontank.
 48. The decoloration method as claimed in claim 38, wherein thefish farming treated in this way is closed circuit farming of seawaterfish.
 49. A plant for decoloring the water of a closed circuit fishfarm, which comprises means for recycling at least part of the water ofat least one rearing pond, means for purifying the recycled water beforeits reuse in the pond in question, new water makeup means for additionto the stream of recycled water, means for ozonating at least part ofthe recycled water and at least part of the new water makeup before itis mixed with the recycled water, said means for ozonating comprising: asingle ozonator as the sole ozonator in the circuit and capable ofsupplying a main flow of ozonated gas; a line for directing apredetermined portion of the main flow of ozonated gas produced by theozonator, through a calibrated orifice, in order to direct a constantflow toward the new water to be ozonated; means for measuring the flowrate of recycled water to be ozonated; and means for adjusting theoutput of said ozonator on the basis of the flow rate of the recycledwater measured by said measuring means.
 50. The plant as claimed inclaim 49, wherein the fish farm treated in this way is a closed circuitfarm for seawater fish.